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Plant Breeding and Predicting Offspring
Traits By Patty Hain -
croptechnology.unl.edu |
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In this lesson students
will plant bicolor sweet corn (sweet corn with yellow and white kernels)
which can be purchased at any garden store. While all the seeds they plant
will be yellow, the kernels of corn on the ears they harvest will be observed
with a ratio of 1 white: 3 yellow. Students will use their
observations to determine the genotype and phenotype of the original parents
and how the traits are passed on to the offspring. Students will also learn
how trait expression is affected by whether an allele is dominant or
recessive and whether an organism is homozygous or heterozygous for that
allele. This experiment may be
continued for a second generation. The kernels harvested from the first
generation can be planted and grown. When the plants reach the proper stage,
students will practice corn breeding techniques to cross (mate) plants. They
will make a series of crosses between plants grown from white and yellow
kernels and also make self crosses. Using Punnett squares students will
predict the genotypic and phenotypic ratios for the offspring of each cross
and compare their predictions with observed results at harvest. |
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Purpose:
The
purpose of this activity is to demonstrate trait expression and how traits are
passed from one generation to the next.
Students will practice corn breeding techniques and use Punnett squares
to predict the phenotypes and genotypes of offspring and their parents.
Objectives (Year 1)
Following completion of this lesson, the students will:
-identify the parts of a corn plant and how they reproduce.
-explain the process of corn breeding.
-demonstrate how traits are passed
to offspring
-identify what gametes are and how their importance in sexual
reproduction.
-define DNA, genes, alleles, and chromosomes and explain
their function in controlling and passing on traits.
-explain how genes control traits, i.e. seed color.
-identify the genotype and phenotype of an organism.
-differentiate between dominant and
recessive alleles
-differentiate between homozygous and heterozygous genotypes
Objectives (Year 2)
-predict the phenoptypic and genotypic ratios of offspring
-determine the genotype of parents based on observations of
offspring.
Materials
1. Seed for bicolor (yellow and white)
kerneled sweet corn. Available at any garden store.
2. Soil
3. Flower pots (12” or larger)
4. Fertilizer
5. Water
6. Warm sunny location (a greenhouse is optimal)
7. Small paper bags
8. Stapler
9. Marker
10. Worksheet
Year
1
Methods
1. Fill pots with soil up to 1” from the top of the pot.
Plants may also be grown outside in a field setting. Check seed packet for
specific planting instructions.
2. Poke 1 hole in soil in center of each pot approximately
1-1½” deep (read seed packet for exact depth).
3. Drop one seed into each hole
4. Gently cover seeds with soil.
5. Record the date the corn was planted for completing the
worksheet.
6. Water seeds until soil is moist
7. Allow plants to grow for several weeks fertilizing as
recommended on fertilizer container. Water regularly (when soil is dry).
8. The rate of plant growth directly depends on the
temperature of its environment. Corn needs a certain number of days to reach
maturity, and this is based on how much heat the plants get. Warmer conditions
may speed up plant growth and shorten the number of days necessary. Similarly,
cooler conditions can lengthen the season. Check the seed packet when purchasing,
and try to choose a variety with the shortest growing season to decrease the
time it takes to complete the experiment.
9. Allow plants to grow to maturity.
When to pick corn:
After the plants begin to tassel, watch for when
approximately half of the plants are showing silks and record this date. The
field will be ready to pick about 18-22 days after the half silk date,
depending upon temperature conditions (source: Earl May Flower & Vegetable
Guide 2002). Kernels should be juicy yet firm.
***NOTE: If you plan to
continue the experiment for a second year, or would like to be able to plant
some of the kernels harvested, allow plants to reach full maturity before
harvesting. The stalks should be dry and brown and kernels will be hard.
Analyzing observations:
Give each student or student group an ear and have them score
and record the data using the worksheet
provided.
Discuss observations and results.
Year 2 (optional)
Follow
planting instructions outlined for year 1. Label each pot (or row if planted in
a field) as to whether the seed planted was yellow or white. Make sure the
labels stay with the plants throughout the entire season.
Corn Breeding:
After several weeks (time depends upon the corn variety) you
will notice an ear shoot developing about halfway up the side of the plant.
Cover the tip of the ear shoot before any silks emerge to prevent
self-pollination. You will also notice a tassel beginning to emerge from the top
of the plant.
Once the
tassel begins to shed pollen place a paper bag over the tassel and grip/fold it
closed over the base of the stem of the tassel to collect any pollen that falls
within the bag. Shake the bag vigorously to loosen any remaining pollen from
the tassel. Carefully bend the top of the stalk over and remove the bag of
pollen. *The silks must be ready to pollinate (emerging from the ear). If they
are not ready when the tassel begins to emerge, put a bag on the tassel to
collect any pollen that falls. Staple the bag closed around the base of the
tassel to keep the pollen within the bag and wait until silks are ready. When
the silks have begun to emerge, shake the tassel bag to remove remaining pollen
from the tassel and dump the bag of pollen onto the silk as outlined in the
next step.
Three different crosses can
be made to demonstrate the possible results.
Cross 1: yellow X yellow (self-cross)
Cross 2: yellow X white (out cross)
Cross 3: white X white (self-cross)
Determine the plant to be
pollinated, remove the bag over the ear (silks should be at least partially
emerged from the ear) and pour the pollen from the bag on the silks. Place the
pollen bag on the ear and staple the base shut to protect the ear from being
contaminated with other pollen. Write on the bag what cross was made (yellow X
white).
For plants that are
self-crossed or self-pollinated, pour the bag of pollen on the ear of the plant
it came from. Label the plant as such. (yellow X
yellow)
Continue to grow the plants
to maturity.
When to pick corn:
Follow harvesting instructions for year 1
***IMPORTANT: label each ear with the cross that was made
(the parents that were mated to make this offspring). A piece of masking tape
wrapped around the ear with the cross written with a permanent marker works
well.
Analyzing observations:
Give each student or student group an ear from each cross and
have them score and record the data using the worksheet provided. Students
should complete the the Year 2 section of the breeding lesson worksheet.
Students and teachers should compare the predicted results with the observed
results from the breeding exercise.
Additional
Articles for Review
National Corn Growers Association – Education resources http://www.ncga.com/education/guide/guide.html
Assessment
Students are assessed based on participation in the
experiment and adequate completion of the worksheets.
WORKSHEET
#1 Trait Inheritance - Observations
NAME_____________________________
Date
planted: _______________________
What was the color of the seed
planted (phenotype)?
A: yellow
Use the letter ‘y’ to denote
this gene. The dominant allele is ‘Y’. The recessive allele is ‘y’.
What are the possible
genotypes of the seed for this trait?
A: YY, Yy, yy We do not know yet if the yellow
allele is dominant or recessive.
From what is observed, can
the genotype and phenotype of the parents that made this seed be determined
yet? Why or why not?
A:
No. Because we do not know if the yellow allele is dominant
or recessive. Therefore we don’t know the genotype of the offspring
(seed planted) and can’t determine the genotype or phenotype of the parents.
Half
silk date: ____________________
Harvest
date: _____________________
Collect
and record data from a single ear:
How
many yellow kernels?
A:
Numbers will vary
How
many white kernels?
A:
Numbers will vary
What
is the ratio of yellow to white kernels?
A:
3 yellow:1 white (may need to round or approximate to
get exact ratio)
Which
allele does this indicate is dominant, yellow or white?
A:
yellow
Use the letter ‘y’ to denote
this gene. The dominant allele is ‘Y’. The recessive allele is ‘y’.
What
would be the genotype(s) of the white kernels? Why?
A:
yy. The white kernel color allele is recessive meaning two copies of the allele
are necessary for expression of the trait.
Would
the genotype(s) be homozygous or heterozygous for this trait?
A:
homozygous
What
would be the genotype(s) of the yellow kernels? Why?
A:
YY and Yy. Since they are yellow in color, they obviously have a Y allele.
However, the fact that some of the offspring were white, yy,
indicates that the parents also had the white allele, y. Therefore, it is possible that some of the
yellow kernels have a YY genotype and others have Yy.
Would
the genotype(s) be homozygous or heterozygous for this trait?
A:
some homozygous, some heterozygous
y
What
would the genotypes of the parents be? Use a Punnett square to determine this
based on the genotypes of the offspring.
y y
yy
A: We know that
the white kernels are homozygous recessive. Therefore each of the parents had
to donate a ‘y’ allele. We can then
deduce that two of the other offspring (out of
four) also received one copy of the ‘y’ allele.
y
y Y
Y YY Yy Yy yy
We also know that he
yellow kernels must each have a yellow
allele
giving them the yellow color. Therefore each of the parents
Had to donate a ‘Y’ allele.
y
Therefore,
the genotypes of both the parents was ‘Yy’,
What
was the phenotype(s) of the parents?
A:
yellow
Can
you determine if a yellow kernel is homozygous or heterozygous by looking at
it?
A:
No
How
might you be able to determine this?
A:
Plant seed from several yellow kernels, self-pollinate them, and look at the
kernels of the progeny. If the kernels
on a plant are all yellow, the original plant and all the offspring (kernels)
are homozygous for the yellow allele. If the kernels have a 3 yellow: 1 white
kernel ratio, then the original plant was heterozygous and the offspring
(kernels) are a mixture of the genotypes.
WORKSHEET
#2 CORN BREEDING – Parts of a corn plant
This
worksheet will have an unlabeled copy of the image of a corn plant mentioned in
the lesson plan above (yet to be drawn).
Students will need to fill in the blanks labeling the different parts of
the corn plant (especially focusing on reproductive parts important for
breeding)
WORKSHEET
#3 Trait Inheritance (year 2) -
Observations
NAME_____________________________
Date
planted: _______________________
Date crossed:
_______________________
Half
silk date:
________________________
Date
harvested:
______________________
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Phenotype |
Phenotype crossed with |
Offspring's phenotype(s) |
Offspring's genotype(s) |
Parent's genotype(s) |
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Parent #1 |
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Parent #2 |
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Parent #3 |
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Parent #4 |
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